Unrefined crude oil stored in a bulk-storage tank has a percentage of water entrained within the oil. Such crude oil is typically pumped into a bulk-storage tank prior to shipment. The capacity of bulk-storage tanks vary, but may be one-hundred (100) million barrels (i.e., 15.9 giga-liters). Over a period of twenty-four (24) to forty-eight (48) hours, the water and oil stored in a tank separate naturally, with the water collecting at the bottom of the tank beneath the oil. The separated water and the crude oil within the tank are very distinct except for a “black water” or “rag” interface layer. The black water interface layer is an emulsion of mixed oil and water.
Prior to transferring the crude oil to a bulk carrier for shipment, the crude oil stored in a bulk-storage tank requires dewatering (i.e., removing the water from the tank). Conventionally, the oil within a bulk-storage tank is dewatered by manually opening an outlet valve at the base of the bulk-storage tank and allowing any contained liquid to run through a pipe to a containment area. The liquid running through the pipe is initially water. An operator periodically checks the liquid, using a siphon point, to see if the liquid is water or oil. The siphon point may be in the form of a domestic tap attached to the pipe. When the operator determines that the liquid has transitioned from water to oil, which occurs after a random time, the operator closes the outlet valve on the bulk-storage tank to stop the flow of liquid. A conventional definition of “transition from water to crude oil” is when a ratio of water to crude oil in the liquid reaches 20:80 (i.e., 20% water: 80% crude oil). The remaining liquid in the tank, which is primarily crude oil, may then be transferred by a separate pipe to a transport system such as a shipping delivery system.
If the operator is unable to stop the flow of liquid from the tank at the point of the liquid transitioning to oil, then oil is sent to the containment area where the oil is trapped in a mixture of oil and waste-water. The oil may be recovered from the waste-water using conventional water processing methods. However, recovering the oil from the containment area is an expensive exercise.
The dewatering of a bulk-storage tank as described above often takes place in open air in extreme environmental conditions such as heat, wind, sand storms, and rain. The reliability and accuracy of such a dewatering method is subject to the diligence of the operators. In particular, the decision point for closure of the outlet valve at the transition of the liquid from water to oil is a subjective judgement and open to vary from one operator to another.
In order to remove the dependence on a human operator for detecting the transition of water to crude oil in a pipe, density sensors have been used to periodically determine density of the liquid within the pipe. One such density sensor is an insertion liquid density transducer (ILDT). An ILDT comprises a tuning fork which is immersed within a pipe in the liquid being measured. The tuning fork is excited into oscillation by a piezoelectric device (not shown) internally secured at the root of one tine. The frequency of the vibration of the tuning fork is detected by a second piezoelectric device secured in the root of the other tine of the tuning fork. The tuning fork is maintained at its natural resonant frequency, as modified by the surrounding liquid, by an amplifier circuit which may be located in an electronic housing. This frequency of vibration is a function of the overall mass of the tine element and the density of the liquid in contact with the tine element. As the density of the liquid changes, the overall vibrating mass changes together with the resonant frequency. By measuring the resonant frequency the density of the liquid can be determined. Another example of a density sensor may be in the form of a tube densitometer. A tube densitometer works in a similar manner to the ILDT discussed above.
The density measurements determined using such density sensors may be used to determine if the transition between water and oil has occurred. In this connection, Tables 3 and 4 of Appendix C show the density of water and crude oil. However, density sensors such as those discussed above are not suitable for use with any liquid of unpredictable or erosive nature which can damage the tines causing erratic results. Further, such density sensors require complicated fitting within a pipe in order to perform the sampling. Still further such density sensors are prone to fouling when sampling particularly viscous liquids such as crude oil.
Thus a need clearly exists for an improved method of detecting water to oil transition of liquid flowing in a pipe.